Energy harvesting device including MEMS composite transducer

ABSTRACT

An energy harvesting device includes a MEMS composite transducer. The MEMS composite transducer includes a substrate. Portions of the substrate define an outer boundary of a cavity. A MEMS transducing member includes a beam having a first end and a second end. The first end is anchored to the substrate and the second end cantilevers over the cavity. A compliant membrane is positioned in contact with the MEMS transducing member. A first portion of the compliant membrane covers the MEMS transducing member. A second portion of the compliant membrane is anchored to the substrate. The compliant member is configured to be set into oscillation by excitations produced externally relative to the energy harvesting device.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, U.S. patent applications Ser.No. 13/089,541, entitled “MEMS COMPOSITE TRANSDUCER INCLUDING COMPLIANTMEMBRANE”, Ser. No. 13/089,532 (now U.S. Pat. No. 8,409,900), entitled“FABRICATING MEMS COMPOSITE TRANSDUCER INCLUDING COMPLIANT MEMBRANE ”,Ser. No. 13/089,500, entitled “ENERGY HARVESTING USING MEMS COMPOSITETRANSDUCER”, all filed concurrently herewith.

FIELD OF THE INVENTION

This invention relates generally to energy harvesting devices, and inparticular to a MEMS-based transducer that converts motion intoelectrical energy.

BACKGROUND OF THE INVENTION

Micro-Electro-Mechanical Systems (or MEMS) devices are becomingincreasingly prevalent as low-cost, compact devices having a wide rangeof applications. Uses include pressure sensors, accelerometers,gyroscopes, microphones, digital mirror displays, microfluidic devices,biosensors, chemical sensors, and others.

MEMS transducers include both actuators and sensors. In other words theytypically convert an electrical signal into a motion, or they convert amotion into an electrical signal. They are typically made using standardthin film and semiconductor processing methods. As new designs, methodsand materials are developed, the range of usages and capabilities ofMEMS devices can be extended.

MEMS transducers are typically characterized as being anchored to asubstrate and extending over a cavity in the substrate. Three generaltypes of such transducers include a) a cantilevered beam having a firstend anchored and a second end cantilevered over the cavity; b) a doublyanchored beam having both ends anchored to the substrate on oppositesides of the cavity; and c) a clamped sheet that is anchored around theperiphery of the cavity. Type c) is more commonly called a clampedmembrane, but the word membrane will be used in a different senseherein, so the term clamped sheet is used to avoid confusion.

Sensors and actuators can be used to sense or provide a displacement ora vibration. For example, the amount of deflection δ of the end of acantilever in response to a stress σ is given by Stoney's formulaδ=3σ(1−ν(L ² /Et ²  (1),where ν is Poisson's ratio, E is Young's modulus, L is the beam length,and t is the thickness of the cantilevered beam. In order to increasethe amount of deflection for a cantilevered beam, one can use a longerbeam length, a smaller thickness, a higher stress, a lower Poisson'sratio, or a lower Young's modulus. The resonant frequency of vibrationof an undamped cantilevered beam is given byf=ω₀/2π=(k/m)^(1/2)/2π,  (2),where k is the spring constant and m is the mass. For a cantileveredbeam of constant width w, the spring constant k is given byk=Ewt ³/4L ³  (3).It can be shown that the dynamic mass m of an oscillating cantileveredbeam is approximately one quarter of the actual mass of ρwtL (ρ beingthe density of the beam material), so that within a few percent, theresonant frequency of vibration of an undamped cantilevered beam isapproximatelyf˜(t/2πL ²) (E/ρ)^(1/2)   (4).For a lower resonant frequency one can use a smaller Young's modulus, asmaller thickness, a longer length, or a larger density. A doublyanchored beam typically has a lower amount of deflection and a higherresonant frequency than a cantilevered beam having comparable geometryand materials. A clamped sheet typically has an even lower amount ofdeflection and an even higher resonant frequency.

Based on material properties and geometries commonly used for MEMStransducers the amount of deflection can be limited, as can thefrequency range, so that some types of desired usages are either notavailable or do not operate with a preferred degree of energyefficiency, spatial compactness, or reliability. For example, usingtypical thin film transducer materials for an undamped cantilevered beamof constant width, Equation 4 indicates that a resonant frequency ofseveral megahertz is obtained for a beam having a thickness of 1 to 2microns and a length of around 20 microns. However, to obtain a resonantfrequency of 1 kHz for a beam thickness of about 1 micron, a length ofaround 750 microns would be required. Not only is this undesirablylarge, a beam of this length and thickness can be somewhat fragile. Inaddition, typical MEMS transducers operate independently. For someapplications independent operation of MEMS transducers is not able toprovide the range of performance desired.

Energy harvesting devices convert ambient energy from the environmentinto electrical energy. An example is a piezoelectric energy harvestingdevice that converts mechanical strain into electric current or voltage.Typically, these devices operate most efficiently when oscillating atmechanical resonance. However, many of the prevalent frequencies ofmotion in the environment tend to be in the low kilohertz range(including pressure waves and mechanical vibrations) down to 100 Hz andbelow (such as vibration from a motor powered at 60 Hz). As discussedabove, a piezoelectric cantilevered beam having a resonant frequency inthis range can be undesirably large and fragile.

Accordingly, there is a need for a MEMS transducer design and method ofoperation that provides low cost, compact, or reliable energy harvestingdevices capable of efficiently converting externally producedexcitations to electrical energy especially when the excitations are ina frequency range that is below a few kilohertz.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an energy harvesting deviceincludes a MEMS composite transducer. The MEMS composite transducerincludes a substrate. Portions of the substrate define an outer boundaryof a cavity. A MEMS transducing member includes a beam having a firstend and a second end. The first end is anchored to the substrate and thesecond end cantilevers over the cavity. A compliant membrane ispositioned in contact with the MEMS transducing member. A first portionof the compliant membrane covers the MEMS transducing member. A secondportion of the compliant membrane is anchored to the substrate. Thecompliant member is configured to be set into oscillation by excitationsproduced externally relative to the energy harvesting device.

According to another aspect of the invention, an energy harvestingapparatus includes an energy harvesting device, a rectifier; and anenergy storage device. The energy harvesting device includes a MEMScomposite transducer. The MEMS composite transducer includes asubstrate. Portions of the substrate define an outer boundary of acavity. A MEMS transducing member includes a beam having a first end anda second end. The first end is anchored to the substrate and the secondend cantilevers over the cavity. A compliant membrane is positioned incontact with the MEMS transducing member. A first portion of thecompliant membrane covers the MEMS transducing member. A second portionof the compliant membrane is anchored to the substrate. The compliantmember is configured to be set into oscillation by excitations producedexternally relative to the energy harvesting device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the example embodiments of the inventionpresented below, reference is made to the accompanying drawings, inwhich:

FIG. 1A is a top view and FIG. 1B is a cross-sectional view of anconfiguration of a MEMS composite transducer including a cantileveredbeam and a compliant membrane over a cavity;

FIG. 2 is a cross-sectional view similar to FIG. 1B, where thecantilevered beam is deflected;

FIG. 3 is a top view of an embodiment of an energy harvesting deviceincluding a MEMS composite transducer and associate circuitry;

FIG. 4A is a cross-sectional view of the MEMS composite transducer ofFIG. 3 in its undeflected state;

FIG. 4B is a cross-sectional view of the MEMS composite transducer ofFIG. 3 in its deflected state;

FIG. 5 is a top view of an embodiment of an energy harvesting deviceincluding a MEMS composite transducer and associate circuitry;

FIGS. 6A and 6B are cross-sectional views similar to FIG. 4A, but wherea mass is affixed to the compliant membrane;

FIG. 7A is a cross-sectional view of the MEMS composite transducerhaving an affixed mass in its undeflected state;

FIG. 7B is a cross-sectional view of the MEMS composite transducerhaving an affixed mass in its deflected state;

FIG. 8 is a cross-sectional view showing additional structural detail ofa MEMS composite transducer including a cantilevered beam that can beused in an energy harvesting device;

FIG. 9 is a top view of an embodiment of an energy harvesting deviceincluding an array MEMS composite transducers having different resonantfrequencies and associate circuitry;

FIG. 10 shows oscillation intensity curves versus driving frequency foran array of MEMS composite transducers having different resonantfrequencies;

FIG. 11A shows an energy harvesting device affixed to a vibratingobject;

FIG. 11B shows an energy harvesting device oriented toward thepropagation direction of pressure waves; and

FIG. 12 is a block diagram describing an example embodiment of a methodof harvesting energy using a MEMS composite transducer.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art.

Embodiments of the present invention include a variety of types of MEMStransducers including a MEMS transducing member and a compliant membranepositioned in contact with the MEMS transducing member, configured suchthat the MEMS composite transducer can be set into oscillation byexcitations such as vibrations or pressure waves in gases, liquids, orsolids, in order to convert such periodic excitation into electricalenergy. The vibrations can be transmitted to the MEMS compositetransducer through, for example, direct or indirect mechanical contactwith a vibrating body, or through sound wave propagation. It is to benoted that in some definitions of MEMS structures, MEMS components arespecified to be between 1 micron and 100 microns in size. Although suchdimensions characterize a number of embodiments, it is contemplated thatsome embodiments will include dimensions outside that range.

Some general characteristics of a MEMS composite transducer will beexplained prior to describing embodiments of the present invention thatinclude a MEMS composite transducer configured as part of an energyharvesting device. FIG. 1A shows a top view and FIG. 1B shows across-sectional view (along A-A′) of a first configuration of a MEMScomposite transducer 100, where the MEMS transducing member is acantilevered beam 120 that is anchored at a first end 121 to a firstsurface 111 of a substrate 110. Portions 113 of the substrate 110 definean outer boundary 114 of a cavity 115. In the example of FIGS. 1A and1B, the cavity 115 is substantially cylindrical and is a through holethat extends from a first surface 111 of substrate 110 (to which aportion of the MEMS transducing member is anchored) to a second surface112 that is opposite first surface 111. Other shapes of cavity 115 arecontemplated for other configurations (not shown) in which the cavity115 does not extend all the way to the second surface 112. Still otherembodiments are contemplated where the cavity shape is not cylindricalwith circular symmetry. A portion of cantilevered beam 120 extends overa portion of cavity 115 and terminates at second end 122. The length Lof the cantilevered beam extends from the anchored end 121 to the freeend 122. Cantilevered beam 120 has a width w₁ at first end 121 and awidth w₂ at second end 122. In the example of FIGS. 1A and 1B, w₁=w₂,but in some embodiments that need not be the case. MEMS transducershaving an anchored beam cantilevering over a cavity are well known. Afeature that distinguishes the MEMS composite transducer 100 from aconventional device is a compliant membrane 130 that is positioned incontact with the cantilevered beam 120 (one example, of a MEMStransducing member). Compliant membrane includes a first portion 131that covers the MEMS transducing member, a second portion 132 that isanchored to first surface 111 of substrate 110, and a third portion 133that overhangs cavity 115 while not contacting the MEMS transducingmember. In a fourth region 134, compliant membrane 130 is removed suchthat it does not cover a portion of the MEMS transducing member near thefirst end 121 of cantilevered beam 120, so that electrical contact canbe made as is discussed in further detail below. In the example shown inFIG. 1B, second portion 132 of compliant membrane 130 that is anchoredto substrate 110 is anchored around the outer boundary 114 of cavity115. In other embodiments (not shown), it is contemplated that thesecond portion 132 would not extend entirely around outer boundary 114.

The portion (including end 122) of the cantilevered beam 120 thatextends over at least a portion of cavity 115 is free to move relativeto cavity 115. A common type of motion for a cantilevered beam is shownin FIG. 2, which is similar to the view of FIG. 1B at highermagnification, but with the cantilevered portion of cantilevered beam120 deflected upward away by a deflection δ=Δz from the originalundeflected position shown in FIG. 1B (the z direction beingperpendicular to the x-y plane of the surface 111 of substrate 110).Such a bending motion is provided for example in an actuating mode by aMEMS transducing material (such as a piezoelectric material, or a shapememory alloy, or a thermal bimorph material) that expands or contractsrelative to a reference material layer to which it is affixed when anelectrical signal is applied, as is discussed in further detail below.When the upward deflection out of the cavity is released (by stoppingthe electrical signal), the MEMS transducer typically moves from beingout of the cavity to into the cavity before it relaxes to itsundeflected position. Some types of MEMS transducers have the capabilityof being driven both into and out of the cavity, and are also freelymovable into and out of the cavity.

Desirable properties of compliant membrane 130 are that it have aYoung's modulus that is much less than the Young's modulus of typicalMEMS transducing materials, that it have a relatively large elongationbefore breakage, and that it have excellent chemical resistance (forcompatibility with MEMS manufacturing processes). Some polymers,including some epoxies, are well adapted to be used as a compliantmembrane 130. Examples include TMMR liquid resist or TMMF dry film, bothbeing products of Tokyo Ohka Kogyo Co. The Young's modulus of cured TMMRor TMMF is about 2 GPa, as compared to approximately 70 GPa for asilicon oxide, around 100 GPa for a PZT piezoelectric, around 160 GPafor a platinum metal electrode, and around 300 GPa for silicon nitride.Thus the Young's modulus of the typical MEMS transducing member is atleast a factor of 10 greater, and more typically more than a factor of30 greater than that of the compliant membrane 130. A benefit of a lowYoung's modulus of the compliant membrane is that this type of designallows for the compliant membrane to have negligible effect on theamount of deflection for the portion 131 of the compliant membrane thatcovers the MEMS transducing member, but is readily deflected in theportion 133 of compliant membrane 130 that is nearby the MEMStransducing member but not directly contacted by the MEMS transducingmember. In addition, the elongation before breaking of cured TMMR orTMMF is around 5%, so that it is capable of large deflection withoutdamage. Furthermore, because the Young's modulus of the compliantmembrane 130 is much less than that of the typical MEMS transducingmember, it has little effect on the resonant frequency of the MEMScomposite transducer 100 if the MEMS transducing member (for example,cantilevered beam 120) and the compliant membrane 130 have comparablesize. However, if the MEMS transducing member is significantly smallerthan the compliant membrane 130, the resonant frequency of the MEMScomposite transducer can be significantly lowered.

Providing a lower resonant frequency MEMS composite transducer is afeature that can be particularly beneficial in an energy harvestingdevice. As mentioned above, piezoelectric energy harvesting devicesconvert mechanical strain into electric current or voltage. Typicallysuch devices operate most efficiently when oscillating at mechanicalresonance. However, many of the prevalent frequencies of motion in theenvironment tend to be in the low kilohertz range (including soundwaves, other pressure waves and mechanical vibrations) down to 100 Hzand below (such as vibration from a motor powered at 60 Hz). To obtain aresonant frequency of 1 kHz or lower for a piezoelectric cantileveredbeam thickness of about 1 micron, a beam length of around 750 microns orlonger would be required. Not only is this undesirably large, a beam ofthis length and thickness can be somewhat fragile.

Referring to FIGS. 3-11B, example embodiments of MEMS compositetransducers 100 suitable for use in an energy harvesting device areshown. This includes different configurations within the family of MEMScomposite transducers 100 having one or more cantilevered beams 120 asthe MEMS transducing member covered by the compliant membrane 130. Thedifferent embodiments within this family have different amounts ofdisplacement or different resonant frequencies or different amounts ofcoupling between multiple cantilevered beams 120 extending over aportion of cavity 115, and thereby are well suited to a variety ofapplications. For an energy harvesting device, cantilevered beams 120having a length that is small in comparison with a dimension across thecavity can be advantageous, in that the compliant membrane 130 can beset into oscillation at a lower resonant frequency, thereby causingdeflection of the cantilevered beams 120. FIG. 3 shows a portion of anenergy harvesting device 200 including a MEMS composite transducer 100having four cantilevered beams 120 as the MEMS transducing members, eachcantilevered beam 120 including a first end that is anchored tosubstrate 110, and a second end 122 that is cantilevered over cavity115. In the embodiment of FIG. 3, the length L of the cantilevered beams120 (the distances from anchored first ends 121 to free second ends 122)are less than 20% of the dimension D across cavity 115. In thisparticular example, where the outer boundary 114 of cavity 115 iscircular, D is the diameter of the cavity 115. In this example, thewidths w₁ (see FIG. IA) of the first ends 121 of the cantilevered beams120 are all substantially equal to each other, and the widths w₂ (seeFIG. 1A) of the second ends 122 of the cantilevered beams 120 are allsubstantially equal to each other. In addition, w₁=w₂ in the example ofFIG. 3. Compliant membrane 130 includes first portions 131 that coverthe cantilevered beams 120 (as seen more clearly in the cross-sectionalview through two cantilevered beams 120 in FIG. 4A), a second portion132 that is anchored to substrate 110, and a third portion 133 thatoverhangs cavity 115 while not contacting the cantilevered beams 120.Portions 134 of the compliant membrane are removed provide access toelectrical contacts to the cantilevered beams 120.

FIG. 4A shows a cross-sectional view through two of the cantileveredbeams 120 and compliant membrane 130 of FIG. 3 in an undeflected state.FIG. 4B shows a similar cross-sectional view, but where the compliantmembrane 130 has been deflected (for example, set into oscillation by anexternal excitation). The amplitude of the deflection of compliantmembrane 130 near the center of cavity 115 is δ=Δz along a directionthat is parallel to axis 117 of cavity 115. The deflected compliantmembrane 130, being in contact with the cantilevered beams 120, causesthe cantilevered beams 120 also to deflect upward. When the oscillatingcompliant membrane 130 subsequently deflects downward into cavity 115,the cantilevered beams 120 are caused to deflect downward. Even a lowfrequency oscillation can cause a short MEMS transducing member having ahigh resonant frequency to deflect because of the coupling of motionfrom the compliant membrane 130 of the MEMS composite transducer 100.Without compliant membrane 130 coupling the oscillation to thecantilevered beams 120, cantilevered beams 120 are not deflectedappreciably by the low frequency external excitation.

For a cantilevered beam 120 including a piezoelectric material, thealternating upward and downward deflection of compliant membrane 130results in alternating compression and tension within the cantileveredbeams 120, thereby generating an AC voltage. As illustrated in FIG. 3,the AC output of an energy harvesting device 200 is typically connectedto a rectifier 210 to provide a DC voltage. The output of rectifier 210is optionally smoothed by a filter 220, and is connected to an energystorage device 230, such as a capacitor or a battery. Optionally, aregulator (not shown) can be provided between the filter 220 and theenergy storage device 230. Because the power output of a piezoelectricenergy harvesting device is typically on the order of microwatts tomilliwatts, it is typically well-suited for providing a trickle chargeto energy storage device 230. Energy storage device 230 is used to poweran electrical device 250. In some embodiments rectifier 210 isintegrated onto the same substrate 110 as MEMS composite transducer 100,while in other embodiments rectifier 210 can be packaged as a hybridcircuit in the same microelectronic package as MEMS composite transducer100. Similarly, in some embodiments, any or all of filter 220, energystorage device 230 and electrical device 250 can also be integrated ontothe same substrate 110, or they can be packaged as a hybrid circuit, orthey can be connected with wires. In some applications, it can beparticularly desirable to package energy harvesting device 200,rectifier 210, filter 220, energy storage device 230 and electricaldevice 250 together in a single microelectronic package, so that nowiring harness is required for electrical connection. Energy harvestingdevice 200, rectifier 210, optional filter 220 (and optional regulator)and energy storage device 230 are together considered to be part of anenergy harvesting apparatus. Electrical device 250 is shown connected toenergy storage device 230 by a dashed line, because it is using energy,not harvesting energy, so it is not part of the energy harvestingapparatus, even though in some embodiments it can be integrated togetherwithin the same substrate.

FIG. 5 shows an embodiment of an energy harvesting device 200 similar toFIG. 3 in which there are two groups of cantilevered beams 120 and 125,with the elements of the two groups being alternatingly arranged. In theembodiment of FIG. 5 the lengths L and L′ of the cantilevered beams 120and 125 respectively (the distances from anchored first ends 121 to freesecond ends 122) are less than 20% of the dimension D across cavity 115.In this particular example, where the outer boundary 114 of cavity 115is circular, D is the diameter of the cavity 115. In addition, in theembodiment of FIG. 6, the lengths L and L′ are different from eachother, the first widths w₁ and w₁′ are different from each other, andthe second widths w₂ and w₂′ are different from each other for thecantilevered beams 120 and 125. Such an embodiment can be beneficial ifthe groups of both geometries of cantilevered beams 120 and 125 are usedto convert a motion of compliant membrane 130 to an electrical signal,and it is desired to pick up different amounts of deflection or atdifferent frequencies. While the embodiment of FIG. 5 includes eightcantilevered beams 120 and 125, other embodiments (not shown) caninclude many more cantilevered beams.

In the embodiments shown in FIGS. 1A, 3 and 5, the cantilevered beams120 (one example of the MEMS transducing members) are disposed widthsubstantially radial symmetry around a circular cavity 115. This can bea preferred type of configuration in many embodiments, but otherembodiments are contemplated having nonradial symmetry or noncircularcavities.

Some embodiments of MEMS composite transducer 100 for an energyharvesting device include an attached mass 118, as shown in FIGS. 6A and6B, in order to further lower the resonant frequency. The mass 118 canbe attached to the portion 133 of the compliant membrane 130 thatoverhangs cavity 115 but does not contact the MEMS transducing member,for example. In the embodiment shown in the cross-sectional view of FIG.6A including a plurality of cantilevered beams 120 (such as theconfigurations shown in FIGS. 3 to 5), mass 118 extends below portion133 of compliant membrane 130, so that it is located within the cavity115. Alternatively, mass 118 can be affixed to the opposite side of thecompliant membrane 130, as shown in FIG. 16B. The configuration of FIG.6A can be particularly advantageous if a large mass is needed. Forexample, a portion of silicon substrate 110 can be left in place as mass118 when cavity 115 is etched. In such a configuration, mass 118 wouldtypically extend the full depth of the cavity 115. In order for the MEMScomposite transducer to vibrate without crashing of mass 118, substrate110 would typically be mounted on a mounting member 180 including arecess 185 below cavity 115 as shown in FIGS. 7A and 7B. FIG. 7A shows across-sectional view with compliant membrane 130 deflected upward alongaxis 117 of cavity 115, while FIG. 7B shows a similar cross-sectionalview with compliant membrane 130 deflected downward along axis 117 intocavity 115, such that mass 118 extends within recess 185 of mountingmember 180. For the configuration shown in FIG. 6B, the attached mass118 can be formed by patterning an additional layer over the compliantmembrane 130. In other embodiments there can be an attached mass 118 onboth sides of the compliant membrane.

In some embodiments, for example, as shown in FIGS. 6A and 6B, mass 118is disposed at or near the center of compliant membrane 118. Further, insome embodiments, mass 118 is disposed symmetrically about a center ofcompliant membrane 130. In such cases, the mass 118 can also be disposedat the center of compliant membrane 130, but in other embodiments themass 118 is not disposed at the center of the compliant membrane.Different distributions of mass 118 attached to compliant membrane 130can facilitate excitation of different vibrational modes havingdifferent resonant frequency.

A variety of transducing mechanisms and materials can be used in theMEMS composite transducer for the energy harvesting device of thepresent invention. In the examples described above, the MEMS transducingmechanisms include a deflection out of the plane of the undeflected MEMScomposite transducer, for example, a bending motion. For a bendingmotion, it is advantageous in some embodiments to include in the MEMStransducing member a piezoelectric MEMS transducing material 160 incontact with a reference material 162, as shown for the cantileveredbeam 120 in FIG. 8. In the example of FIG. 8, the MEMS transducingmaterial 160 is shown on top of reference material 162, butalternatively the reference material 162 can be on top of the MEMStransducing material 160. Reference material 162 can include one or moreinsulator layers such as silicon oxide or silicon nitride, as well asone or more conductor layers to serve as electrodes. It is preferable todesign the relative thicknesses t₁ and t₂ of the piezoelectric MEMStransducing material 160 and the reference material 162 such that theneutral axis (on either side of which stress and strain change sign) isnot within the MEMS transducing material 160. In other words, it isdesired for efficiency of conversion from strain to voltage in thepiezoelectric material that when the cantilevered beam 120 is deflectedupward, the piezoelectric MEMS transducing material either be completelyin compression or completely in tension throughout the cantilevered beam120. Similarly, when the cantilevered beam 120 is deflected downward, itis desired that the piezoelectric MEMS transducing material either becompletely in tension or completely in compression throughout thecantilevered beam 120.

Piezoelectric materials are particularly advantageous for use in MEMScomposite transducer for an energy harvesting device, because of theirability to convert a strain into an electrical signal. There are avariety of types of piezoelectric materials. A family of interestincludes piezoelectric ceramics, such as lead zirconate titanate or PZT.

Because there can be a wide range of frequencies of mechanical vibrationor pressure waves in the environment, it is advantageous to provide anenergy harvesting device 200 that is effective in converting a widerange of frequencies of mechanical or pressure wave excitation intoelectrical energy. One way to broaden the frequency response is to adddamping into oscillating MEMS composite transducers. However, that oftenreduces the amplitude of oscillation near resonance. A preferredapproach is to provide an array of MEMS composite transducers havingdifferent resonant frequencies, but a relatively low amount of damping.It is preferable for the damping of oscillation of the MEMS compositetransducer of the energy harvesting device to be due primarily toconversion of mechanical energy into electrical energy, rather than toinclude additional sources of mechanical damping. In some embodiments,the damping of oscillations of the MEMS composite transducer in anenergy harvesting device is reduced by enclosing the device in ahermetically sealed housing 205 (shown in FIG. 11A) from which air hasbeen removed.

FIG. 9 shows a portion of an energy harvesting device 200 including alower frequency MEMS composite transducer 101, a middle frequency MEMScomposite transducer 102 and a higher frequency MEMS compositetransducer 103 that are all formed together on a single substrate 110.Each of the MEMS composite transducers is similar in that they include acavity 115, at least one cantilevered beam 120, a compliant membrane130, and optionally a mass 118 attached to the compliant membrane 130.Cavities 115 for MEMS composite transducers 101, 102 and 103 havediameters D₁, D₂ and D₃ respectively, where D₁>D₂>D₃. In other words,lower frequency MEMS composite transducers tend to have a largerdiameter of cavity 115. Lower frequency MEMS composite transducers alsotend to have a larger mass 118 affixed to compliant membrane 130. Inaddition, in the example shown in FIG. 9, the length L of cantileveredbeam 120 from its anchored first end to its free second end is longerfor MEMS composite transducer 101 than it is for MEMS compositetransducers 102 or 103. Also shown in FIG. 9 are the rectifier 210,filter 220, and energy storage device 230 that are part of thecorresponding energy harvesting apparatus, as well as the electricalenergy device 250 that is powered thereby.

Energy harvesting devices 200 are not limited to arrays of MEMScomposite transducers having three different resonant frequencies.Depending upon the range of frequencies that are desired to convert toelectrical energy in a particular application, an energy harvestingdevice 200 can include a plurality of MEMS composite transducers havingdifferent resonant frequencies, thereby providing a wider range ofvibration frequency sensitivity than would an energy harvesting devicenot including a plurality of MEMS composite transducers having differentresonant frequencies.

FIG. 10 schematically shows three intensity curves as a function ofdriving frequency (where intensity is the square of the amplitude ofoscillation) for three different lightly damped MEMS compositetransducers, having different resonant frequencies. For example, curve301 (corresponding to MEMS composite oscillator 101) has a resonancenear lower frequency f₁, curve 302 (corresponding to MEMS compositeoscillator 102) has a resonance near middle frequency f₂, and curve 303(corresponding to MEMS composite oscillator 103) has a resonance nearhigher frequency f₂. The intensity curves 301, 302 and 303 overlapsomewhat, so even for excitations that are not at one of the resonantfrequencies, a reasonable amount of electrical signal can be generatedin the energy harvesting device 200 and used to charge energy storagedevice 230 or power electrical device 250 (see, for example, FIG. 9).

Referring to FIG. 12, having described the components of an energyharvesting device and an energy harvesting apparatus, a context isprovided for describing a method of harvesting energy from theenvironment. At least one MEMS composite transducer 100 is provided,step 400, including a substrate 110 having a cavity 115 with at leastone cantilevered beam 120 and a compliant membrane 130 in contact withthe cantilevered beam 120 and anchored to the substrate. The energyharvesting device is configured so that the compliant membrane 130 isset into oscillation by excitations produced external to the energyharvesting device, step 405. Such excitations could include mechanicalvibrations of an object, or pressure waves (including sound waves, orother types of pressure pulses in fluids). The oscillating compliantmembrane 130 moves the cantilevered beam 120 into and out of cavity 115,step 410. Because cantilevered beam 120 includes a piezoelectricmaterial, the motion of cantilevered beam is converted into anelectrical signal, step 415. More particularly, an AC electrical signalis provided as the cantilevered beam 120 is deflected into and out ofcavity 115, thereby alternatingly being put into tension andcompression. The AC electrical signal is rectified by rectifier 210, andoptionally filtered by filter 220. The rectified signal is typicallyused to provide a trickle charge to an energy storage device 230, whichcan be a capacitor or a battery, for example.

Because the power output of an energy harvesting device is typically onthe order of microwatts to milliwatts, such an energy harvesting deviceis particularly well-suited for providing power to electrical devicesthat require low amounts of energy, or that are intermittently operated.Smoke detectors and camera flashes are examples of devices that areintermittently operated. An energy harvesting device can be used tocharge an energy storage device for such an intermittently operateddevice. Although a relatively high amount of energy is required when thedevice is operated, use can be infrequent enough so that energyharvesting device can sufficiently charge the energy storage devicebetween operations.

FIG. 11A shows an energy harvesting device 200 including a plurality ofMEMS composite transducers 100 of the types described earlier relativeto FIGS. 3-9. Axis 117 of cavities 115 is indicated. Mounting member 180(see FIGS. 7A and 7B) is not shown but can be affixed to housing 205. Insome embodiments, housing 205 can be hermetically sealed and a partialvacuum provided within housing 205 for reduction of damping ofoscillations of the MEMS composite transducers. Energy harvesting device200 is in mechanical contact with an object 260 that tends to vibrate.Mechanical contact can be provided by affixing the energy harvestingdevice 200 to object 260 using adhesives, screws, clamps, or the like.Preferably the energy harvesting device 200 is affixed to the object 260in a predetermined relative orientation. For example, the predeterminedrelative orientation can be selected according to a known vibration modeof the object 200 having an axis 265 of vibration that is substantiallyperpendicular to a plane in which MEMS composite transducer 100 isdisposed. Equivalently, it can be said that the predetermined relativeorientation can be selected according to a known vibration mode of theobject 200 having an axis 265 of vibration that is substantiallyparallel to axis 117 of cavity 115.

FIG. 11B shows an energy harvesting device 200 including a plurality ofMEMS composite transducers 100 of the types described earlier relativeto FIGS. 3-9. Axis 117 of cavities 115 is indicated. Pressure waves 270are shown traveling along propagation direction 275 toward energyharvesting device 200. Pressure waves 270 can include sound waves orother sorts of longitudinal waves (also called compression waves) thatcan be propagated through a gas (such as air) or a liquid (such as wateror blood). It is desirable to configure energy harvesting device 200 sothat compliant membrane 130 of MEMS composite transducer 100 is set intooscillation by the pressure waves. In particular, it is preferable toorient the energy harvesting device 200 in a predetermined orientation,such that axis 117 of the cavity 115 is oriented along a direction thatis substantially parallel to a propagation direction 275 of the pressurewaves 270. Other components of the energy harvesting apparatus(rectifier, filter, regulator and energy storage device) are not shownin FIGS. 11A and 11B.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   -   100 MEMS composite transducer    -   101 Lower frequency MEMS composite transducer    -   102 Middle frequency MEMS composite transducer    -   103 Higher frequency MEMS composite transducer    -   110 Substrate    -   111 First surface of substrate    -   112 Second surface of substrate    -   113 Portions of substrate (defining outer boundary of cavity)    -   114 Outer boundary    -   115 Cavity    -   117 Axis of cavity    -   116 Through hole (fluid inlet)    -   118 Mass    -   120 Cantilevered beam    -   121 Anchored end (of cantilevered beam)    -   122 Cantilevered end (of cantilevered beam)    -   125 Cantilevered beam    -   130 Compliant membrane    -   131 Covering portion of compliant membrane    -   132 Anchoring portion of compliant membrane    -   133 Portion of compliant membrane overhanging cavity    -   134 Portion where compliant membrane is removed    -   160 MEMS transducing material    -   162 Reference material    -   180 Mounting member    -   185 Recess    -   200 Energy harvesting device    -   205 Housing    -   210 Rectifier    -   220 Filter    -   230 Energy storage device    -   250 Electrical device    -   260 Vibrating object    -   265 Axis of vibration (of vibrating object)    -   270 Pressure waves    -   275 Propagation direction    -   301 Intensity curve for lower resonant frequency    -   302 Intensity curve for middle resonant frequency    -   303 Intensity curve for higher resonant frequency    -   400 Provide an energy harvesting device including a MEMS        composite transducer    -   405 Oscillate compliant membrane of composite transducer using        excitations produced externally to the energy harvesting device    -   410 Oscillation of compliant membrane causes movement of MEMS        transducing member of composite transducer into and out of the        cavity    -   415 Convert motion of MEMS transducing member into an electrical        signal.

The invention claimed is:
 1. An energy harvesting device comprising: aMEMS composite transducer comprising: a substrate, portions of thesubstrate defining an outer boundary of a cavity; a MEMS transducingmember including a beam having a first end and a second end, the firstend being anchored to the substrate and the second end beingcantilevered over the cavity; and a compliant membrane positioned incontact with the MEMS transducing member, a first portion of thecompliant membrane covering the MEMS transducing member, and a secondportion of the compliant membrane being anchored to the substrate, thecompliant member being configured to be set into oscillation byexcitations produced externally relative to the energy harvestingdevice.
 2. The energy harvesting device of claim 1, wherein thecompliant membrane is anchored to the substrate around the outerboundary of the cavity.
 3. An energy harvesting device comprising: aMEMS composite transducer comprising: a substrate, portions of thesubstrate defining an outer boundary of a cavity; a MEMS transducingmember including a beam having a first end and a second end, the firstend being anchored to the substrate and the second end beingcantilevered over the cavity; and a compliant membrane positioned incontact with the MEMS transducing member, a first portion of thecompliant membrane covering the MEMS transducing member, and a secondportion of the compliant membrane being anchored to the substrate, thecompliant member being configured to be set into oscillation byexcitations produced externally relative to the energy harvestingdevice, the outer boundary including a dimension across the cavity,wherein a distance between the first end and the second end of the beamis less than 20% of the dimension across the cavity.
 4. The energyharvesting device of claim 3, the MEMS transducing member being thefirst of a plurality of MEMS transducing members each comprising a beamanchored to the substrate at a first end and cantilevering over thecavity at a second end.
 5. The energy harvesting device of claim 4, eachof the beams including a corresponding length between its first end andits second end, wherein the lengths of the plurality of beams aresubstantially equal to each other.
 6. The energy harvesting device ofclaim 1, wherein a shape of the cavity is substantially cylindrical. 7.The energy harvesting device of claim 1, the MEMS transducing memberbeing freely movable into and out of the cavity.
 8. The energyharvesting device of claim 1, wherein the MEMS transducing membercomprises a piezoelectric material.
 9. The energy harvesting device ofclaim 8, wherein the piezoelectric material comprises a piezoelectricceramic.
 10. The energy harvesting device of claim 9, wherein thepiezoelectric ceramic comprises lead zirconate titanate.
 11. The energyharvesting device of claim 1, wherein the compliant membrane comprises apolymer.
 12. The energy harvesting device of claim 11, wherein thepolymer comprises an epoxy.
 13. The energy harvesting device of claim 1,the MEMS transducing member having a first Young's modulus and thecompliant membrane having a second Young's modulus, wherein the firstYoung's modulus is at least 10 times greater than the second Young'smodulus.
 14. The energy harvesting device of claim 1, the MEMS compositetransducer being one of a plurality of similar MEMS compositetransducers.
 15. The energy harvesting device of claim 14, each of thesimilar MEMS composite transducers including a cavity having a dimensionacross an outer boundary of the cavity, wherein a dimension across theouter boundary of the cavity of a first MEMS composite transducer isdifferent from a corresponding dimension across the outer boundary of asecond MEMS composite transducer.
 16. The energy harvesting device ofclaim 14, each of the similar MEMS composite transducers including abeam having a length from its first end to its second end, wherein thelength of the beam of a first MEMS composite transducer is differentfrom a length of the beam of a second MEMS composite transducer.
 17. Theenergy harvesting device of claim 1 further comprising a rectifier. 18.The energy harvesting device of claim 1, being configured to provide atrickle charge to an energy storage device.
 19. The energy harvestingdevice of claim 1, being configured to provide electrical power to anelectrical device without requiring a wiring harness.
 20. The energyharvesting device of claim 1 further comprising a mass attached to thecompliant membrane.
 21. The energy harvesting device of claim 20,wherein the mass is disposed at a center of the compliant membrane. 22.The energy harvesting device of claim 20, wherein the mass issymmetrically disposed about a center of the compliant membrane.
 23. Theenergy harvesting device of claim 1, the compliant membrane having afirst side in contact with the MEMS transducing member and a second sideopposite the first side, the energy harvesting device further comprisinga mass affixed to the first side of the compliant membrane.
 24. Theenergy harvesting device of claim 23, the substrate and the mass eachcomprising silicon.
 25. An energy harvesting apparatus comprising: anenergy harvesting device comprising: a MEMS composite transducercomprising: a substrate, portions of the substrate defining an outerboundary of a cavity; a MEMS transducing member including a beam havinga first end and a second end, the first end being anchored to thesubstrate and the second end being cantilevered over the cavity; and acompliant membrane positioned in contact with the MEMS transducingmember, a first portion of the compliant membrane covering the MEMStransducing member, and a second portion of the compliant membrane beinganchored to the substrate, the compliant member being configured to beset into oscillation by excitations produced externally relative to theenergy harvesting device; a rectifier; and an energy storage device. 26.The energy harvesting apparatus of claim 25, wherein the energy storagedevice is a capacitor.
 27. The energy harvesting apparatus of claim 25,wherein the energy storage device is a battery.
 28. The energyharvesting apparatus of claim 25 further comprising a filter.